The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to an image forming apparatus of the type developing a latent image electrostatically formed on an image carrier with a viscous and dense developing liquid.
Image forming apparatuses of the type described are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-152254, 7-209922 and 7-219355. In the apparatuses taught in these documents, charging means uniformly charges the surface of a photoconductive element. Writing means exposes the charged surface of the element in accordance with image data in order to electrostatically form a latent image. A developing device includes a reservoir or tank storing a viscous and dense developing liquid and develops the latent image with the liquid to thereby produce a corresponding toner image. The developing liquid is implemented by a carrier liquid and toner dispersed in the carrier liquid with a high content. The carrier liquid is dimethyl polysiloxane or similar insulative liquid. The developing liquid is adjusted to have a viscosity as high as 100 Pa.multidot.s to 10,000 Pa.multidot.s.
The developing liquid is applied to the surface of a developing roller or that of a developing sleeve included in the developing device, forming a thin layer of uniform thickness. The developing roller, for example, conveys the developing liquid to a developing region where it faces the photoconductive element. At the developing region, the developing liquid develops the latent image formed on the photoconductive element to thereby produce a corresponding toner image. The developing liquid left on the developing roller is removed by a blade and then collected in the reservoir. The toner image is transferred from the photoconductive element to a paper or similar recording medium and then fixed on the recording medium by a fixing device. The toner remaining on the photoconductive element after the image transfer is removed by cleaning means.
Specifically, in the developing device taught in the above Laid-Open Publication No. 7-209922, a double-gear pump is disposed in a reservoir storing a developing liquid. A developing roller is located outside of and above the reservoir. The double-gear pump pumps the developing liquid and applies it to the developing roller. As a result, the liquid forms a thin layer on the developing roller.
In the developing device proposed in the above Laid-Open Publication No. 7-219355, a draw-up roller draws up a developing liquid stored in a reservoir. A regulating roller applies the developing liquid to a developing roller located outside of the reservoir. A screw or a rotatable or pivotable fin is disposed in the reservoir for agitating the developing liquid.
However, each of the conventional developing devices has some problems left unsolved, as follows. Because the developing roller is positioned outside of the reservoir, a broad opening is present on the top of the reservoir. As a result, when the developing device is caused tilt or shake by accident, the developing liquid runs out of the reservoir. This problem is particularly serious when a full-color image is formed by a plurality of developing liquids of different colors. To solve this problem, the reservoir has customarily been provided with a sufficiently great size relative to the amount of the developing liquid to be stored therein. This, however, increases the overall size of the image forming apparatus.
When the developing liquid consisting of the insulative carrier liquid and toner is left unused over a long period of time, it is likely that the carrier liquid and toner are separated from each other or that the toner distribution becomes irregular. Further, because the toner or the carrier liquid is sequentially consumed due to repeated development, a fresh carrier liquid or fresh toner must be replenished in order to readjust the toner content of the developing liquid. The prerequisite with such replenishment is that the developing liquid in the reservoir and the toner or the carrier liquid replenished be uniformly mixed together. However, it is not easy to uniformly mix the dense developing liquid having a viscosity as high as 100 Pa.multidot.S to 10,000 Pa.multidot.S. As a result, the toner cannot be uniformly dispersed in the carrier liquid. Moreover, the developing liquid left on the photoconductive element after development is also collected in the reservoir and must be mixed with the developing liquid present in the reservoir as uniformly as the toner or the carrier liquid replenished. It is, however, extremely difficult to uniformly disperse the toner of the collected developing liquid in the carrier liquid because the toner content of the collected developing liquid has varied in accordance with the image area ratio of a latent image based on image data.
Even when the screw, rotatable or pivotable fin or similar agitating member taught in Laid-Open Publication No. 7-219355 agitates the developing liquid, not the entire liquid but only the liquid around the agitating member moves. Should the developing liquid with an uneven toner content develop a latent image, the resulting toner image would be low quality.